Turning RSUs into a retirement corpus: 401(k), backdoor and mega-backdoor Roth for US employees

A senior engineer at a US tech company earns $230,000 in salary and vests another $180,000 in RSUs a year. On paper that is a high income. But the part that quietly builds a retirement corpus is not the salary or the shares — it is how much of it gets routed into tax-advantaged accounts before the IRS takes its cut on the rest. Done well, RSU cash flow lets you fill every retirement bucket the tax code allows. Done passively, it becomes a pile of taxable single-stock risk and a half-empty 401(k).

The 30-second answer: RSU shares cannot go into a 401(k), but RSU cash can fund the contributions. Sell vested RSUs (tax-neutral at vest) to cover living costs, which frees your salary to max the 2026 buckets: the 401(k) employee deferral of $24,500, the mega-backdoor Roth using after-tax 401(k) space up to the $72,000 Section 415(c) limit, and a backdoor Roth IRA of $7,500. High earners are phased out of direct Roth IRA contributions, so the backdoor is standard; and from 2026, if your 2025 FICA wages topped $150,000, any age-50 catch-up must be Roth. Fill tax-advantaged space first, taxable brokerage second.

This guide is for US residents who want to convert equity compensation into a durable retirement corpus rather than a concentrated stock bet. It covers the order of operations, the 2026 limits, the backdoor and mega-backdoor Roth mechanics, and where the leftover RSU money should go. The vest-taxation mechanics themselves live in the US residents RSU pillar; here we assume you already sell at vest and focus on where the cash goes next.

The core idea: RSUs are a cash-flow bridge, not a retirement asset

You cannot contribute company shares to a 401(k) or an IRA. Only cash, and in the 401(k)'s case only cash from payroll deferral, can go in. So the lever RSUs give you is indirect but powerful: the proceeds from selling vested RSUs cover your spending, which lets you divert a far larger share of your salary into tax-advantaged accounts than your take-home pay alone could support.

Concretely: instead of living on your post-401(k) paycheck, you crank your payroll deferrals up to the maximum, take home less, and backfill your monthly spending from RSU sale proceeds. The RSUs become the bridge that lets you fully fund accounts you otherwise could not afford to max. The tax you pay is unchanged — you owed ordinary income tax on the vest regardless — but the destination of the money is transformed.

The reason this matters is the asymmetry of compounding. A dollar diverted into a Roth account in your thirties and left for thirty years at an assumed 7% nominal return becomes roughly $7.61 — entirely tax-free on withdrawal. The same dollar spent today buys one dollar of lifestyle. RSU proceeds let you convert near-term spending power, which you have in surplus during high-vest years, into long-dated tax-sheltered growth that you cannot buy back later once the contribution window for a given year closes. Each year's tax-advantaged limits are use-it-or-lose-it.

Worked example. How much tax-advantaged space the bridge unlocks

Take the senior engineer from the opening: $230,000 salary, $180,000 in RSUs, single filer, under 50, and a plan that supports the mega-backdoor with a $11,500 employer contribution. Without RSUs, maxing every bucket would mean diverting more than a third of gross salary into accounts they cannot touch, which few households can absorb. With RSU proceeds covering living costs, the same person can fill all of it:

Bucket	2026 contribution
401(k) employee deferral	$24,500
Employer match and contributions	$11,500
After-tax 401(k) (mega-backdoor)	$36,000
Backdoor Roth IRA	$7,500
Total into tax-advantaged accounts	$79,500

That $79,500 a year is the real output of the bridge. Of it, $43,500 (the after-tax 401(k) plus the backdoor Roth) lands in Roth where it grows tax-free forever; $36,000 (deferral plus match) is pre-tax and grows tax-deferred. None of this requires earning more — only redirecting salary that RSU proceeds replace.

Bottom line: RSUs do not go into a 401(k), but RSU cash lets a high earner route roughly $79,500 a year into tax-advantaged accounts they otherwise could not afford to fund.

The order of operations for 2026

Before the order of operations, here are the 2026 numbers you are working against. Keep this table handy; the rest of the guide refers back to it.

2026 limit	Amount
401(k) employee elective deferral	$24,500
Age-50 catch-up (total $32,500)	$8,000
Super catch-up, ages 60–63 (total $35,750)	$11,250
Section 415(c) total annual additions	$72,000
Section 415(c) including age-50 catch-up	$80,000
IRA contribution limit (traditional or Roth)	$7,500

The Section 415(c) figure is the ceiling on everything that lands in your 401(k) in a year — your deferral, the employer match, and any after-tax contributions combined. It is the number that governs how much mega-backdoor room you have, so it is worth committing to memory.

Fill these in order. Each tier assumes you have RSU cash available to replace the salary you are diverting.

1. 401(k) up to the employer match

Always capture the full match first — it is an immediate, risk-free return on contribution. If your employer matches 50% on the first 6%, that is free money no other account offers.

2. Max the 401(k) employee deferral — $24,500

The 2026 elective deferral limit is $24,500. If you are 50 or older, you can add an $8,000 catch-up ($32,500 total); ages 60 to 63 get a super catch-up of $11,250 ($35,750 total). Note the SECURE 2.0 rule below: high earners' catch-ups must now be Roth.

3. Backdoor Roth IRA — $7,500

Almost every RSU holder at a US company earns above the Roth IRA income phase-out, so you cannot contribute to a Roth IRA directly. The backdoor route: contribute up to $7,500 to a traditional IRA on a non-deductible basis, then convert it to a Roth IRA. Because the contribution was already after-tax, the conversion is usually tax-free.

4. Mega-backdoor Roth — up to the $72,000 ceiling

If your plan supports it, this is the largest single lever. The 2026 Section 415(c) limit on all contributions to your 401(k) is $72,000. Subtract your $24,500 deferral and your employer's contributions; the remainder is after-tax space you can fill and then convert to Roth.

5. Taxable brokerage

Once the tax-advantaged buckets are full, remaining RSU proceeds go into a taxable account — ideally broad index funds, which are tax-efficient and where you can harvest losses.

Worked example. A full year of cash flow, with RSUs backfilling living expenses

This is where the bridge becomes concrete. The same single filer earns $230,000 in salary and sells $180,000 of RSUs at vest. We will assume a blended federal-plus-state marginal rate of roughly 35% on ordinary income for simplicity (your actual rate depends on your state and brackets). The point is to trace where each dollar goes, not to nail a tax return.

Start with salary. Out of the $230,000, the employee deferral and after-tax 401(k) contributions come out of payroll before the money ever reaches the bank account:

Salary side	Amount
Gross salary	$230,000
Less: 401(k) deferral (pre-tax)	−$24,500
Less: after-tax 401(k) contributions	−$36,000
Salary remaining before tax	$169,500
Less: estimated tax on salary (~35% of $205,500 taxable)	−$71,925
Take-home salary	≈ $97,575

Note the deferral reduces taxable salary to $205,500, while the after-tax 401(k) is funded with already-taxed dollars, so it does not reduce the tax bill — it is simply diverted before it reaches your checking account. Take-home salary lands near $97,575.

Now the RSUs. The $180,000 vest is taxed as ordinary income at vest (employer usually withholds around 22% to 37%; assume the same ~35% effective marginal rate nets out to roughly $117,000 after tax if sold immediately at vest with no further gain or loss):

RSU side	Amount
RSU vest value	$180,000
Less: tax on vest (~35%)	−$63,000
Net RSU cash	≈ $117,000

Combine the two cash streams and fund the IRA:

Combined household cash	Amount
Take-home salary	$97,575
Net RSU cash	$117,000
Total spendable cash	$214,575
Less: backdoor Roth IRA contribution	−$7,500
Cash available for living + taxable investing	$207,075

The household now lives on $207,075 of after-tax cash — comfortably more than its $97,575 take-home salary alone — precisely because RSU proceeds backfilled what the aggressive payroll deferrals took out. And across the year it has moved $24,500 + $36,000 + $7,500 = $68,000 into tax-advantaged accounts from its own contributions, plus the $11,500 employer match, for $79,500 of total annual funding. Whatever of the $207,075 is not spent flows into the taxable brokerage as the diversified counterweight to any remaining company stock.

Bottom line: the order is match, deferral, backdoor Roth, mega-backdoor, then taxable — and RSU proceeds are what make a take-home this low survivable while $79,500 a year goes to work tax-advantaged.

The mega-backdoor Roth, in detail

This is the play that separates employees who quietly build seven-figure Roth balances from those who do not, and it is entirely about whether your specific plan allows it.

Two conditions must both be true:

1. Your 401(k) plan allows after-tax (non-Roth, non-traditional) contributions beyond the $24,500 deferral.
2. Your plan allows either in-plan Roth conversions or in-service withdrawals so you can move those after-tax dollars into Roth before they generate much taxable growth.

If both hold, here is the math.

Worked example. Mega-backdoor Roth space

Your 2026 numbers:

• Section 415(c) total limit: $72,000
• Your employee deferral: $24,500
• Employer match and contributions: $11,500

After-tax room = $72,000 − $24,500 − $11,500 = $36,000.

You contribute that $36,000 after-tax through payroll, then immediately convert it to Roth (in-plan or via in-service rollover to a Roth IRA). Only the contribution amount is converted with no tax; if you let it sit and it earned, say, $400 before converting, that $400 of growth is taxable on conversion — which is why you convert quickly. Funded annually, this single move puts an extra $36,000 of tax-free-growth money to work every year, on top of the $24,500 deferral and the $7,500 backdoor Roth. The RSU proceeds are what make the reduced take-home pay survivable.

If your plan does not offer after-tax contributions or in-service conversion, the mega-backdoor is simply unavailable — check your plan document or ask HR for "after-tax contributions and in-plan Roth conversion," not "mega-backdoor Roth," which is an informal name they may not recognise.

Worked example. What the mega-backdoor compounds to over a decade

A single year of mega-backdoor funding is useful; the case for it is the repetition. Assume you fund $36,000 of after-tax 401(k) and convert it to Roth every year, and assume a 7% nominal return. The 7% is an assumption, not a promise — markets do not deliver a smooth annual return, and your real outcome will be higher or lower. With that caveat, the Roth balance built from this one lever alone looks like this:

Years funded at $36,000/yr	Total contributed	Roth value at 7%
5	$180,000	≈ $221,000
10	$360,000	≈ $531,000
15	$540,000	≈ $966,000
20	$720,000	≈ $1,576,000

The figures assume contributions at the start of each year and growth compounding annually. By year 20 you have put in $720,000 and the Roth holds roughly $1.58 million — and every dollar of that, including the roughly $856,000 of growth, comes out tax-free in retirement. In a taxable account, that same $856,000 of gain would face capital-gains tax on withdrawal. That tax saving, compounded and never paid, is the entire reason the mega-backdoor is worth the administrative hassle of doing the conversion each year.

Bottom line: the mega-backdoor only works if your plan allows after-tax contributions plus in-plan conversion, but where it does, funding $36,000 a year for two decades builds a roughly $1.58 million Roth balance whose growth is never taxed.

The SECURE 2.0 Roth catch-up change you need to know for 2026

Starting January 1, 2026, if you are 50 or older and your 2025 FICA wages from your employer exceeded $150,000, any catch-up contribution you make must be on a Roth (after-tax) basis rather than pre-tax. RSU vest income counts as FICA wages, so high-comp employees cross this threshold routinely.

What it means in practice:

• Your catch-up dollars lose the upfront tax deduction but grow tax-free thereafter.
• If your plan does not offer Roth deferrals at all, affected high earners cannot make catch-up contributions until the plan is amended. Plans must adopt the amendment by the end of 2026.
• For most high earners this is a mild positive over a long horizon — tax-free growth on the catch-up usually beats a one-year deduction.

To make the threshold concrete: a 52-year-old who earned $250,000 in 2025 FICA wages is over the $150,000 line, so their entire 2026 catch-up — up to $8,000 — must go in as Roth. They lose the upfront deduction worth roughly $2,800 at a 35% marginal rate, but the $8,000 then compounds tax-free. Over fifteen years at an assumed 7%, that single $8,000 becomes about $22,100, all withdrawable tax-free, against the one-time $2,800 deduction they gave up. For a long runway the trade favours Roth; for someone retiring in two or three years the lost deduction matters more.

Bottom line: if your 2025 FICA wages topped $150,000, your 2026 age-50 catch-up must be Roth — you lose the deduction but gain tax-free growth, which is a net positive for anyone more than a few years from retirement.

Asset location: put the right asset in the right account

Filling the buckets is half the job. What you hold inside each account is the other half, and getting it right adds return with no extra risk.

• Roth accounts (Roth IRA, mega-backdoor Roth): hold your highest-expected-growth assets here — broad equity index funds, and any individual growth bets you keep. All future growth is tax-free, so you want the assets that grow the most.
• Traditional / pre-tax 401(k): hold bonds and other income-producing assets that would otherwise be taxed at ordinary rates in a taxable account. Sheltering ordinary-income assets here is the most efficient use of pre-tax space.
• Taxable brokerage: hold tax-efficient broad index funds, where qualified dividends and long-term gains are taxed at preferential rates and you can harvest losses. This is also the natural home for the diversified proceeds of your RSU sales.

Worked example. Naive versus optimal placement of the same three assets

Suppose you hold $300,000 split equally across three sleeves — $100,000 in a high-growth equity fund, $100,000 in a bond fund, and $100,000 in a broad index fund — and you have three accounts of $100,000 each: a Roth, a pre-tax 401(k), and a taxable brokerage. The asset mix is identical in both scenarios below; only the placement changes.

Asset	Naive placement	Optimal placement	Why
High-growth equity	Taxable	Roth	Highest expected growth shelters tax-free; no tax ever on the gains
Bond fund	Roth	Pre-tax 401(k)	Ordinary-income interest hides in pre-tax space instead of being taxed yearly
Broad index fund	Pre-tax 401(k)	Taxable	Qualified dividends and long-term gains already get preferential rates; losses are harvestable

The naive version does the opposite of what each account is good for. It wastes precious tax-free Roth room on bonds that grow slowly, parks the fastest-growing asset in a taxable account where every eventual gain is taxed, and traps a tax-efficient index fund inside pre-tax space where its withdrawals will later be taxed as ordinary income. The optimal version puts the highest-growth asset where growth is never taxed, the ordinary-income asset where its interest is sheltered, and the already-tax-efficient asset where preferential rates and loss harvesting apply.

The dollar gain depends on returns and your tax rates, but the direction is unambiguous: over a few decades, correct asset location commonly adds a few tenths of a percent per year to after-tax return — on $300,000 that compounds into tens of thousands of dollars, earned purely by where you put each fund rather than by taking any extra risk.

A common mistake is holding the same balanced fund in every account. Splitting by asset location — growth in Roth, bonds in pre-tax, index funds in taxable — can add a meaningful amount over a multi-decade horizon for free.

Bottom line: put your highest-growth assets in Roth, ordinary-income assets like bonds in pre-tax, and tax-efficient index funds in taxable — same risk, more after-tax return, for free.

Edge cases

• The pro-rata rule. A backdoor Roth is only clean if you have no other pre-tax IRA balances (traditional, SEP, SIMPLE). If you do, the IRS pro-rates the conversion and part becomes taxable. The fix is often to roll existing pre-tax IRA money into your 401(k) first, if the plan accepts it.

Worked example. The pro-rata rule on a backdoor Roth

You make a $7,500 non-deductible contribution to a traditional IRA, intending to convert it to Roth tax-free. But you also have a $50,000 pre-tax balance in a rollover IRA from an old job. The IRS does not let you cherry-pick the after-tax dollars. Instead it treats all your IRAs as one pool and pro-rates the conversion:

• Total IRA balance after the contribution: $50,000 + $7,500 = $57,500.
• After-tax (non-deductible) portion: $7,500, or 13.04% of the pool.
• Pre-tax portion: $50,000, or 86.96% of the pool.

When you convert $7,500, only 13.04% of it — about $978 — is treated as the after-tax money and converts tax-free. The other $6,522 is treated as pre-tax and is taxable on conversion. At a 35% marginal rate, that is roughly $2,283 of unexpected tax on what you thought was a clean backdoor. The fix: before doing the backdoor, roll that $50,000 pre-tax balance into your current employer's 401(k) (if the plan accepts incoming rollovers). That empties the IRA pool of pre-tax money, leaving only the $7,500 after-tax contribution, so the full conversion is tax-free again.

• Leaving before year-end. Job changes mid-year can leave you having over-contributed across two employers' plans. Track your year-to-date deferrals across both.
• You will return to India later. If you are an Indian citizen who may move back, Roth and 401(k) balances raise cross-border questions — US early-withdrawal penalties, and how India taxes these accounts for a returning resident. That is a separate analysis; see the returning-NRI guide linked below before assuming your US retirement accounts travel cleanly.
• Near-term goals. Do not lock money you need within a few years into retirement accounts to chase the tax break. Liquidity for a house or a sabbatical comes first.

Bottom line: clear pre-tax IRA balances into your 401(k) before running a backdoor Roth to avoid the pro-rata tax, track deferrals across mid-year job changes, and keep near-term cash liquid rather than locked away.

The closing read

The employees who build real retirement wealth from equity comp are not the ones who picked the best stock. They are the ones who treated every RSU vest as cash to fill tax-advantaged space — match, then the $24,500 deferral, then the backdoor Roth, then the mega-backdoor to the $72,000 ceiling, then taxable — year after year, while holding the right asset in each account. The shares are volatile and concentrated. A Roth balance compounding tax-free for thirty years is neither. Use the first to build the second while you have the income to do it.

Cross-references

• US residents with US RSUs: the complete tax and strategy guide
• What to do with vested RSUs: the diversification playbook
• State tax optimization for US RSU holders
• What happens to your 401(k) and IRA when you return to India
• Should you sell RSUs at vest or hold? A decision framework
• Sell-to-cover, sell-all or hold: the three RSU vest decisions
• ESPP vs RSU: how to think about both
• What is an RSU? Restricted Stock Units explained

Critical disclaimer: this article reflects US federal retirement-plan rules and IRS contribution limits announced for 2026 as of June 2026, and is general information, not personalised advice. Contribution limits, the SECURE 2.0 Roth catch-up threshold, plan-specific features (after-tax contributions, in-plan conversions), and the pro-rata rule depend on your plan document and personal facts. Cross-border treatment for those who may leave the US is materially different. Consult a licensed CPA or CFP before acting.